orthodeoxia

Fall in arterial blood oxygen upon assuming the upright posture. Usually caused by right-to-left cardiac or vascular shunting with a posturally induced fall in left-sided pressure permitting a corresponding gradient across the shunt.

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or·tho·de·ox·ia (or″tho-de-okґse-ə) accentuation of arterial hypoxemia in the erect position, improved by assumption of a recumbent position.